Keratin-like proteins are present in the following structural elements within mammalian sperm heads: the nuclear chromatin, perforatorium, and perinuclear material. Within the tail, such proteins are localized in the outer mitochondrial membranes, dense fibers and fibrous sheat. They acquire insolubility in sodium docecyl sulfate as a result of the oxidation of thiols to form disulfide crosslinks, largely during epididymal transit. Concomitant reductions in the permeability of sperm structures cross-linked by disulfide bridges seem to occur, and will be correlated, if possible, with changes in water content and density. Preliminary evidence suggests that 2 plus, important constituent of rat spermatozoa, resides primarily in association with -SH groups in these cells and protects the sperm thiol groups from oxidation by trace metals (e.g. Cu 2 plus). The influence of Zn 2 plus and Cu 2 plus oxidation of -SH during the maturation and aging spermatozoa will be further evaluated by in vitro studies performed with material from rats, rabbits and humans. The role of Zn 2 plus in the biosynthesis and assembly of thiol-rich proteins within the nuclei and tails of rat spermatozoa will be studied by classical biochemical methods (e.g. isolation and characterization of proteins, in vitro protein synthesis experiments) supplemented by measurements of the ultrastructural localization and concentration of this ion. Isolation of spermatids may be necessary for the biosynthetic studies, which will be carried out under varying conditions of temperature and glucose concentration. The effects of androgens and drugs on the behavior of this system will also be evaluated.